Gold is recovered from its ores by leaching with alkaline cyanide solution, forming the aurocyanide ion Au(CN).sub.2.sup.- in solution. Concentrations are generally very low, typically a few parts per million (ppm) or less. The gold is typically adsorbed from solution by activated carbon, and is in turn removed from the carbon by elution with a caustic solution. The caustic solution, now relatively concentrated in gold, is treated by electrowinning to recover elemental gold.
One of the disadvantages of carbon is the complexity and cost of its regeneration after caustic elution. Precipitation of carbonates tend to clog the carbon pores, and adsorption of traces of organic materials can deactivate its gold adsorption properties. To restore the carbon's activity, it is treated with a mineral acid solution and then roasted. During each cycle of the roasting process, a significant portion of the carbon mass is lost, resulting in decreasing particle diameters. When the particles become so small that they risk being lost to the leaching system along with their loaded gold, they must be removed and discarded.
An alternative to carbon adsorption is adsorption on an ion exchange resin. With ion exchange resins, the gold can be recovered by simple caustic elution; further regeneration steps are not needed. To accomplish this, the reactive functionality of the resin must be pH-dependent; that is, capable of adsorbing aurocyanide over the pH range typical of cyanide leach solutions--normally pH 9 to 11--and also capable of eluting aurocyanide in the pH range of eluant solutions--normally above pH 12. This means the functionality must be in its cationic form during extraction and neutral during elution. One of the few functionalities capable of this specific pH dependency is the guanidine functionality. Guanidine ion exchange resins for recovery of precious metals have been described in U.S. Pat. Nos. 4,895,597, 5,028,259 & 5,198,021.
It is desirable in an ion exchange system for gold recovery that the rate and extent of elution of gold be as great as possible. Slow elution rates mean that the resin must spend a larger portion of its cycle time in the elution process rather than extracting more gold, and the equipment for conducting the elution must be larger. To the extent that gold elution is not complete, gold will be carried with the eluted resin back to extraction so that its ultimate recovery is delayed. Furthermore, residual gold on eluted resin may result in less complete extraction of gold from the leach solution.
A significant means for increasing the rate of gold elution from ion exchange resins is described in U.S. Pat. No. 5,198,021, in which the combination of an alkali metal hydroxide and an alkali metal salt of an aliphatic or aromatic carboxylic acid is used to elute gold. It would be desirable to be able to enhance the rate of gold elution still further, allowing the size of elution equipment to be decreased and permitting more reliable operation of the overall gold recovery system.